Loudspeaking telephones (also known as speakerphones or hands free telephones) are ones that locate a microphone and a loudspeaker outside of a conventional telephone handset in somewhat close proximity to each other, thereby creating the potential for sustained oscillation to occur. This situation, known as singing, is often encountered in public address systems when signals from a loudspeaker are too closely coupled to an associated microphone. Loudspeaking telephones generally include amplifiers in both the transmit and receive paths of a telephone set as well as a hybrid circuit that interfaces the transmit and receive channels (comprising 4 wires) to a 2-wire telephone line. Although the hybrid circuit couples most of the transmitted signal energy to the telephone line, a small portion finds its way back into the receive channel. This small portion is attenuated by what is known as trans-hybrid loss. In a similar manner a small portion of the acoustic energy emanating from the loudspeaker is picked up by the transmitting microphone. This small portion is attenuated by what is known as electro-acoustic loss. Thus a loop is created comprising the transmit channel and receive channel coupled by trans-hybrid loss at one end and electroacoustic loss at the other. Oscillation occurs when the net gain around the loop exceeds unity (0 dB).
Perhaps the earliest technique used to circumvent the oscillation problem was the so-called push-to-talk system. In its normal state the transmit channel is disabled and the receive channel is enabled. When a user wants to talk he depresses a manual switch to enable the transmit channel and simultaneously disable the receive channel. Oscillation could never occur because the transmit and receive channels are never on at the same time.
An improvement in the push-to-talk system came when the manual switch is replaced by circuitry that detected speech energy at the transmitter which was thereafter used to enable the transmit channel and disable the receive channel--a technique known as voice switching. A refinement of the voice-switched system came with the inclusion of circuitry to compare the magnitude of the transmit and receive signals and enable the loudest talker to dominate. Apart from the dubious wisdom of rewarding such behavior is the general problem of clipping syllables during the time that directions of transmission are being changed. Prior art circuits frequently combine a 40 dB amplifier with a series switch in the transmit and receive channels; when transmit switch "S1" is on, receiver switch "S2" is off, and vice versa.
Switching 20 dB (10:1) of gain in T seconds, for example, creates about as much noise as switching 40 dB (100:1) of gain in 10 T seconds since it switches the signal 0.1 times as far. To soften switching noise (electrical transients resulting from capture and/or sudden background level noise changes), attack time control circuitry is added. This often results in speech clipping (missed initial voice signals), especially if thee signals are soft as with "h" an "s" sounds. To prevent the loop from oscillating at switching speeds, long time constants, hysteresis, and an offset bias in favor of the transmit channel are built into the loop. When no signals are present, the circuit slowly returns to the condition whereby the transmit channel is enabled and the receive channel is disabled. Some prior art circuits replace S1 and S2 with variolossers to turn the respective channels on and off slowly and in a balanced manner. Other prior art designs control the variolosser in a manner that permits conversation to flow in both directions simultaneously at levels proportional to the volume of the speaking parties. Examples of such techniques are disclosed in U.S. Pat. No. 3,889,059 issued to Thompson et al, U.S. Pat. No. 3,953,676 issued to Brown, and U.S. Pat. No. 4,513,177 issued to Nishino et al. These techniques are generally expensive and are made complex by an algorithm based on a direct comparison between the relative volumes of the transmit and receive signal levels. Furthermore, it is not clear that such "see-saw" action provides benefits commensurate with the cost of providing them.
It is therefore an object of the present invention to provide voice-switched operation in a loudspeaking telephone that minimizes speech clipping in a cost effective manner.
It is another object of the present invention to eliminate the direct comparison of transmit and receive signals in determining transmit channel and receive channel amplification.